In the fabrication of integrated circuits, various conductive layers are used. For example, during the formation of semiconductor devices, such as dynamic random access memories (DRAMs), or any other types of memory devices, conductive materials are used in the formation of storage cell capacitors, and also may be used in interconnection structures, e.g., conductive layers in contact holes, vias, etc. For example, in the fabrication of integrated circuits including capacitor structures, conductive layers are used for capacitor electrodes. Memory circuits, such as DRAMs and the like, use conductive structures to form opposing electrodes of storage cell capacitors.
As memory devices become more dense, it is necessary to decrease the size of circuit components forming such devices. One way to retain storage capacity of storage cell capacitors of the memory devices, and at the same time decrease the memory device size, is to increase the dielectric constant of the dielectric layer of a storage cell capacitor. Therefore, high dielectric constant materials are used in such applications and interposed between two electrodes. One or more layers of various conductive materials may be used as the electrode material. Generally, one or more of the layers of conductive material used for the electrodes (particularly the bottom electrode of the cell capacitor) has certain diffusion barrier properties, e.g., silicon or oxygen diffusion barrier properties. Such properties are particularly required when high dielectric constant materials are used for the dielectric layer of the storage cell capacitor because of the processes used in forming such high dielectric constant materials, e.g., deposition of high dielectric constant materials usually occurs at high temperatures (generally, greater than about 500° C.) in an oxygen-containing atmosphere.
Various metals and metallic compounds, for example, metals such as platinum, and conductive metal oxides such as ruthenium oxide, have been proposed as the electrode materials or at least one of the layers of an electrode stack for use with high dielectric constant materials. However, electrodes generally need to be constructed such that they do not diminish the beneficial properties of the high dielectric constant materials. For example, for platinum to function well as a bottom electrode or as one of the layers of an electrode stack, an effective barrier to the diffusion of silicon from the substrate or other silicon-containing region to the top of the electrode needs to be provided. This is typically required since silicon at the surface of the electrode stack will tend to be oxidized during the oxygen anneal of the high dielectric constant materials and/or during deposition of oxide dielectrics, e.g., Ta2O5 or BaSrTiO3, which will result in a decreased series capacitance, thus degrading the storage-capacity of the capacitor. In addition, oxygen diffusion through the platinum electrode layer during high temperature oxidizing processes, e.g., BaSrTiO3 deposition processes, needs to be prevented. Such oxygen diffusion through the platinum occurs generally through the platinum grain boundaries.
Further, during high temperature processing of devices (e.g., high dielectric constant material formation processes) that include platinum conductive layers, stress occurs in the platinum layer. Such stress may result in the formation of a discontinuous platinum layer, such as in the form of platinum islands, which are undesirable. The formation of such platinum islands may result in films that are unstable for use as capacitor electrodes.
In addition to the use of high dielectric constant materials for capacitor structures, it is desirable to take other steps to increase or preserve capacitance without increasing the occupied area. For example, electrode surfaces may be roughened to increase the effective surface area of electrodes without increasing the area occupied by the capacitor.
One method for providing a roughened surface for a plate of a storage cell capacitor is to form the plate of hemispherical grain polysilicon (HSG), possibly with an overlying metal layer. The hemispherical grains of HSG enhance the surface area of the plate without increasing its occupied area.
However, HSG presents difficulties in fabrication because of the formation of silicon dioxide on and near the HSG. A silicon dioxide layer may form on the HSG, particularly during deposition of the capacitor's dielectric layer. Even with an intervening metal layer present, oxygen from the deposition of the dielectric layer can diffuse through the metal layer, forming silicon dioxide at the polysilicon surface. Silicon diffusion through the metal layer may also produce an undesirable silicon dioxide layer between the metal and the dielectric layers.
To avoid these negative effects caused by formation of silicon dioxide, a diffusion barrier layer may be employed between the HSG and the metal layer. But, in a typical capacitor geometry, the greater the total number of layers, the larger the required minimum area occupied by the capacitor. Further, the upper surface of each additional layer deposited over the HSG tends to be smoother than the underlying surface, reducing the increased surface area provided by the HSG.